Charger for hand-held rechargeable electric apparatus with reduced magnetic field

ABSTRACT

An improved system for charging a battery of a hand held device, such as an electric toothbrush. The system has a first coil coupled around a first magnetic core. The first magnetic core forms a substantially closed magnetic path except for a core gap. An AC power signal is applied to the first coil to create a varying magnetic field within the core gap. The core gap is configured to minimize radiation of magnetic flux whether the second magnetic core is in place or not by having its faces closely spaced and aligned substantially parallel to one another. A second coil is coupled around a second magnetic core. The second magnetic core is insertable into the core gap to form an AC current in the second coil. The second magnetic core is sized to substantially occupy the core gap to minimize radiation of magnetic flux. The size of the gap and the length of the second magnetic core are made as small as possible to keep the weight of the hand-held device low. The second coil is coupled to a rectifier which is in turn coupled to charge the battery.

This is a Continuation-In-Part of application Ser. No. 08/269,392, whichwas filed on Jun. 30, 1994, which has been abandoned.

FIELD OF THE INVENTION

This invention relates to electrically-operated hand-held devices. Morespecifically, it relates to hand-held devices operated by rechargeablebatteries in which the recharger is coupled to the hand-held devicethrough a mechanically separable transformer.

BACKGROUND

Hand-held battery-operated devices are well known in the art. Oralhygiene devices in this category, such as electric toothbrushes, arealso well know in the art.

Battery operation is both more convenient and safer than operating fromstandard alternating current (AC) outlets. Devices equipped withrechargeable batteries are even more convenient, because the batteriesdo not have to be replaced periodically.

One well known technique for recharging such batteries is to equip thehand-held device with an electrical connector. A recharger, whichtypically converts the high voltage AC available from an outlet to lowvoltage AC or direct current (DC), is coupled electrically to thisconnector. Devices that employ this approach are disclosed in U.S. Pat.Nos. 4,827,551 and 4,827,552. This technique has two well-knowndisadvantages. First, it requires a hole for the connector through thedevice's enclosure, making it difficult to seal the device's internalparts against water, dirt, solvents, and so forth. Second, it providesan unlikely but still possible electrical connection between the user ofthe hand-held device and the AC outlet.

An improved charger, known in the art, eliminates any electricalconnection between the hand-held device and the AC outlet. Instead, therequired recharging power is coupled magnetically between the rechargerand the hand-held device. The charger contains the primary of atransformer, and the hand-held device contains the secondary of atransformer. The primary and the secondary of the transformer aremechanically separable. When the two are placed in proper orientationand close proximity and the primary is electrically coupled to an ACoutlet, a varying magnetic field is electrically induced in the primaryof the transformer, and is then coupled to the secondary of thetransformer. The secondary coil of the transformer is used to generatecharging current for the rechargeable batteries. This approach allowsthe hand-held device to be completely sealed, and provides no electricalconnection between an AC outlet and the hand-held device.

One device that employs such a charging arrangement is the Interplak®Home Plaque Removal Instrument manufactured by Bausch & Lomb of Tucker,Ga., and described, in part, in the disclosure of U.S. Pat. No.4,845,795. However, such an approach can produce substantial low- andmedium-frequency magnetic fields in the vicinity of the charger.

Although at present there are no widely-accepted limits on humanexposure to magnetic fields, most authorities recommend prudentavoidance. Many authorities especially recommend avoiding exposingchildren to magnetic fields for lengthy periods of time.

Chargers for electric toothbrushes of the type described above are oftenintended to operate continuously. For example, the charger for theInterplak® does not have an on/off switch; it is simply plugged into anoutlet. Such chargers are often placed in bathrooms, and bathrooms areoften adjacent to rooms in which children sleep. Common buildingmaterials such as wood, drywall, and plaster have no effect on thepropagation of magnetic fields. Consequently, it is possible forchildren to be inadvertently and unknowingly exposed, for lengthyperiods of time while they sleep, to magnetic fields emanating from suchprior-art chargers.

Turning to FIG. 1, we see a prior-art electric toothbrush 2 resting in arecess in its prior-art charger/base 1. The charger/base is connected toa power source such as an AC outlet through a cable 6. Through a portionof the charger/base that has been cut away, we see a primary transformerwinding 7, which may be driven directly with the AC current deliveredthrough the cable 6. Alternatively, the primary transformer winding 7may be driven with a higher frequency current generated by an electroniccircuit contained somewhere within the charger/base 1, which in turnreceives power from the cable 6.

Examining the prior-art electric toothbrush 2 of FIG. 1, we see that itincorporates an on/off switch 4. A mechanical activator, such as amotor, inside the toothbrush 2 drives a shaft through the neck 5 of thetoothbrush, which in turn activates the head of the toothbrush.

The head of the toothbrush may be of any type such as those disclosed inU.S. Pat. Nos. 4,827,550, 4,827,551, 4,827,552, or 4,845,796, and is notshown in FIG. 1 or FIG. 2 because the form of the head is not relevantto the present invention.

Inspecting the lower end of the toothbrush 2 in FIG. 1, we see that itrests in a recess formed in the charger/base 1. We see that a portion ofthe toothbrush 2 has been cut away, revealing the bottom end of arechargeable battery 3, a rectifier 9, and a transformer secondary coil9. The transformer secondary coil 9 converts a portion of the varyingmagnetic field, which was formed by a primary alternating currentflowing through the transformer primary coil 7, into a secondaryalternating current. This secondary alternating current is rectified bythe rectifier 9, then is used to recharge the battery 3 contained withinthe toothbrush 2.

Turning now to FIG. 3A, we see the prior art transformer primary coil 7and transformer secondary coil 8 of FIG. 1 more clearly. FIG. 3A, forclarity, has omitted all of the charger/base 1 and toothbrush 2 of FIG.1.

It will be apparent to one familiar with the art that the prior-art formof transformer shown in FIG. 3A forms a substantially open magneticcircuit, and that lines of magnetic flux will radiate outwards from thistransformer in an essentially toroidal form.

Turning now to FIG. 3B, we see a prior art separable transformer thatforms a substantially closed magnetic circuit, thus reducing theradiation of lines of magnetic flux under most circumstances. A primarycore 34, surrounded by a primary winding 33, is magnetically coupled toa separable secondary core 31 and its associated secondary winding 32.It will be apparent to one familiar with the art that this prior art hasan important disadvantage if radiated magnetic fields are a concern.When the separable secondary core 32 is separated from the primary core34, the primary will form an essentially open magnetic circuit and thatmagnetic lines of flux will radiate from the primary faces 35. It willalso be apparent to one familiar with the art that such a pot-core canbe of practical use only at frequencies substantially higher thanpower-line frequencies. Prior art of this type may be found in thedisclosures of U.S. Pat. Nos. 2,967,267, 3,292,579, 3,840,795 (FIGS. 7and 8), 4,038,625, 4,942,352, and 5,157,319.

Turning now to FIG. 3C, we see a prior art separable transformer, with amagnetically permeable core 43 placed coaxially between a primary coil41 and a secondary coil 42. It will be apparent to one familiar with theart that this prior art has an important disadvantage if radiatedmagnetic fields are a concern, because the core does not form a closedmagnetic circuit and therefore allows lines a magnetic flux to radiateoutwards from the primary coil in a toroidal form stretched along theaxis of the core 43. Such prior art separable transformers may be foundin the disclosures of U.S. Pat. Nos. 3,418,552 and 3,840,795 (FIG. 9),and may be found in the Oral-B Plaque Remover™ electric toothbrushmanufactured by Braun, Inc. of Lynnfield, Mass.

Turning now to FIG. 3D, we see a prior art separable transformer thatforms a substantially closed magnetic circuit, thus reducing theradiation of lines of magnetic flux under most circumstances. A primarycore 53, surrounded by a primary winding 51, is magnetically coupled toa separable secondary core 54 and its associated secondary winding 52.It will be apparent to one familiar with the art that this prior art hasan important disadvantage if radiated magnetic fields are a concern.When the separable secondary core 54 is separated from the primary core53, the primary will form an essentially open magnetic circuit and thatmagnetic lines of flux will radiate from the primary faces 55. It willalso be apparent to one familiar with the art that such a secondary core54 may contribute substantial weight to a hand-held device. Such addedweight can make a hand-held device such as a toothbrush cumbersomeand/or perhaps impossible to use for the time period necessary for toothbrushing for small children, the elderly or the infirm. Prior art ofthis type may be found in the disclosures of U.S. Pat. Nos. 3,277,358,3,549,990, 3,772,625, 4,030,058, and 4,912,391.

Turning now to FIG. 3E, we see a prior art separable transformer thatforms a substantially closed magnetic circuit, thus reducing theradiation of lines of magnetic flux under most circumstances. A primarycore 63, surrounded by a primary winding 61, is magnetically coupled toa separable secondary core 64 and its associated secondary winding 62.It will be apparent to one familiar with the art that this prior art hasan important disadvantage if radiated magnetic fields are a concern.When the separable secondary core 64 is separated from the primary core63, the primary will form an essentially open magnetic circuit and thatmagnetic lines of flux will radiate from the primary faces 65. It willalso be apparent to one familiar with the art that such a secondary core64 may contribute substantial weight to a hand-held device. Prior art ofthis type may be found in the disclosures of U.S. Pat. Nos. 3,938,018and 4,146,857.

Turning now to FIG. 3F, we see a prior art separable transformer thatforms a substantially closed magnetic circuit, thus reducing theradiation of lines of magnetic flux under most circumstances. A primarycore 73, surrounded by a primary winding 71, is magnetically coupled toa separable secondary core 74 and its associated secondary winding 72.It will be apparent to one familiar with the art that this prior art hasan important disadvantage if radiated magnetic fields are a concern.When the separable secondary core 74 is separated from the primary core73, the primary will form an essentially open magnetic circuit and thatmagnetic lines of flux will radiate from the primary faces 75. It willalso be apparent to one familiar with the art that such a secondary core74 may contribute substantial weight to a hand-held device. Prior art ofthis type may be found in the disclosures of U.S. Pat. Nos. 2,415,688and 3,641,336.

It is desirable to minimize the weight of the secondary portion of amechanically separable transformer, which is found in the hand-heldportion of the device, in order to minimize hand strain and encourageproper operation of the hand-held device.

It is desirable to minimize the radiated magnetic field emitted by thecharger base when the hand-held device is removed from the charger.

It is an object of the present invention to retain the advantages of amechanically-separable transformer charger arrangement for hand-heldrechargeable-battery operated devices, while substantially reducing thestrength of the low- and medium-frequency magnetic fields produced nearsuch a charger arrangement.

It is a further object of the present invention to minimize the weightof the transformer secondary in such a reduced-field implementation.

It is a further object of the present invention to minimize the magneticflux emitted by the charger base when the hand-held device is removedfrom the base.

SUMMARY OF THE INVENTION

The invention is an improved system for charging a battery of a handheld device, such as an electric toothbrush. The system has a first coilcoupled around a first magnetic core. The first magnetic core forms asubstantially closed magnetic path except for a core gap. An AC powersignal is applied to the first coil to create a varying magnetic fieldwithin the core gap. The core gap is configured to minimize radiation ofmagnetic flux. A second coil is coupled around a second magnetic core.The second magnetic core is insertable into the core gap to form an ACcurrent in the second coil. The second magnetic core is sized tosubstantially occupy the core gap to minimize radiation of magneticflux. The second coil is coupled to a rectifier which is in turn coupledto charge the battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a prior-art electric toothbrush, placedin its recharger/holder, with portions of the toothbrush and itsrecharger/holder cut away to reveal the prior-art mechanically separabletransformer.

FIG. 2 is a perspective view of the present invention, as applied to theprior-art electric toothbrush of FIG. 1, again with portions of thetoothbrush and its recharger/holder cut away to reveal the mechanicallyseparable transformer of the present invention.

FIG. 3A is a perspective view of a prior-art separable transformer,shown removed from the prior-art toothbrush and recharger/holder of FIG.1 for increased clarity.

FIG. 3B is a perspective view of another prior-art separabletransformer, commonly referred to as a pot core transformer.

FIG. 3C is a perspective view of yet another prior-art separabletransformer, commonly referred to as a co-axial core transformer.

FIG. 3D is a perspective view of yet another prior-art separabletransformer, commonly referred to as a `U` core transformer.

FIG. 3E is a perspective view of yet another prior-art separabletransformer, also commonly referred to as a `U` core transformer.

FIG. 3F is a perspective view of yet another prior-art separabletransformer, commonly referred to as an `E` core transformer.

FIG. 4A is a perspective view of an embodiment of the separabletransformer of the present invention, drawn in such a way as toconveniently contrast with the prior art shown in FIG. 3A, and shownremoved from the prior-art toothbrush and recharger/holder of FIG. 2 forincreased clarity.

FIG. 4B is a perspective view of an embodiment of the separabletransformer of the present invention, with the secondary moved a littledistance to more clearly reveal a key element of the present invention.

FIG. 5 is a perspective view of the preferred embodiment of theseparable transformer of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning to FIG. 2, we see an embodiment of the present invention, drawnin such a way as to conveniently contrast with the prior art of FIG. 1.The charger/base 1 and toothbrush 2 are substantially the same as inFIG. 1, but the separable transformer 7, 8 of FIG. 1 has been replacedby a separable transformer of a different magnetic design. This newtransformer consists of a primary winding 24 encircling a `C`-shapedcore 23, and a secondary winding 21 encircling a short, straight sectionof core 22. The short, straight section of core 22 is positioned so thatit substantially fills the gap in the `C`-shaped core 23. The primarywinding 24 and its associated `C`-shaped core 23 are contained withinthe charger/base 1, and the secondary winding 21 and its associated core22 are contained within the toothbrush 2.

Turning now to FIG. 4A, we see the transformer primary coil 24, itsassociated `C`-shaped core 23, the secondary winding 21 and itsassociated core 22 of FIG. 2 more clearly. FIG. 4A for clarity hasomitted all of the charger/base 1 and toothbrush 2 of FIG. 2.

It will be apparent to one familiar with the art that the core 23, 22 ofthe transformer shown in FIG. 4A forms a substantially closed magneticcircuit, and consequently confines the magnetic lines of flux to thepath of the core 22, 23. A minor amount of magnetic flux radiatesoutwards from the gaps between cores 22 and 23.

I have determined by experiment that the magnetic field strength at agiven distance for a transformer of the form shown in FIG. 3B, whencompared with an otherwise identical transformer of the prior-art formshown in FIG. 3A operating under otherwise identical conditions, islower by more than two orders of magnitude.

The cores 22, 23 may be formed of any convenient ferromagnetic material,such as grain-oriented silicon steel or ferrite.

It will be apparent to one familiar with the art that the physicalalignment of the secondary core 22 of FIG. 4A within the gap in theprimary core 23 is moderately critical in that the secondary core 22should substantially span the gap in the primary core 23. This alignmentmay be accomplished with any convenient mechanical means, such asforming a tab in the bottom of the toothbrush 2 of FIG. 2 which mateswith a corresponding slot in the charger/base 1, or by forming thehandle of the toothbrush 2 into an oval cross section and forming therecess in the charger/base 1 into a corresponding oval cross section.

Turning now to FIG. 4B, we see the separable transformer of FIG. 4Aoperating with the its secondary 21, 22 in its separated position, as itwould be, for example, when a toothbrush is being used rather than whenit is resting in its charging base. It will be apparent to one familiarwith the art that lines of magnetic flux will travel directly betweenprimary faces 81, but will balloon outward somewhat in the center of thegap defined by primary faces 81. This provides a marked improvement overthe prior art embodiments shown for example in FIGS. 3D, 3E and 3F.

It should be apparent that the embodiment of the present invention shownin FIGS. 4A and 4B contrasts conveniently with the prior art of FIGS. 1and 3A, but that the advantages of the present invention can best beseen in the embodiment show in FIG. 5.

Turning now to FIG. 5, we see the preferred embodiment of the presentinvention. A separable transformer is formed of a primary coil 94,primary core 93, secondary coil 91, and secondary core 92. The primarycore 93 and secondary core 92 together form a closed magnetic path, thusminimizing the radiation of magnetic lines of flux. The weight of thesecondary 91, 92 is minimized by keeping the secondary core 92 as shortas practical. In the preferred embodiment, the secondary core 92 is lessthan 1/2" long. Keeping the secondary core 92 short has the additionaladvantage of minimizing the distance between the faces 95 of the primarycore, thus minimizing the radiation of magnetic flux when the secondary91, 92 is separated for use. The primary core 93 and the secondary core92 may be formed of any low reluctance material, such as laminatedgrain-oriented silicon steel.

The present invention has been described in terms of specificembodiments incorporating details to facilitate the understanding of theprinciples of construction and operation of the invention. Suchreference herein to specific embodiments and details thereof is notintended to limit the scope of the claims appended hereto. It will beapparent to those skilled in the art that modifications may be made inthe embodiment chosen for illustration without departing from the spiritand scope of the invention. Specifically, it will be apparent that toone of ordinary skill in the art that the device of the presentinvention could be implemented in several different ways and theapparatus disclosed above is only illustrative of the preferredembodiment of the invention and is in no way a limitation.

What is claimed is:
 1. A system including a first electric circuit fortransferring AC electric power to a second electric circuit which ismounted within a portable hand-held device such that the second electriccircuit and the device are separable from the first electric circuit,wherein there is no electrical connection between the first electriccircuit and the second electric circuit, wherein the first electriccircuit comprises a first coil coupled around a first magnetic core suchthat the first coil is coupled to receive an AC power signal for forminga varying magnetic field,and wherein the second circuit comprises asecond magnetic coil coupled around a second magnetic core for receivingthe varying magnetic field, the system wherein the first magnetic coreforms a substantially closed magnetic path having a magnetic core gap,such magnetic core gap having a first gap face and a second gap facewhich are aligned to each other, and the second magnetic core fitswithin and is substantially aligned with the magnetic core gap at thosetimes when the second magnetic circuit is magnetically coupled to thefirst magnetic circuit such that the radiation of the magnetic flux isminimized when the second magnetic core is aligned and also whenseparated from the first magnetic core.
 2. The second circuit accordingto claim 1 further comprising a rectifier coupled to the coil and abattery coupled to the rectifier for storing power.
 3. The secondcircuit according to claim 2 wherein the size of the core gap isminimized to reduce the radiation of magnetic flux and wherein thesecond magnetic core is sufficiently large to accommodate sufficientwindings of the second coil to charge the battery.
 4. The second circuitaccording to claim 3 wherein the core gap is substantially 1/2 inch. 5.The system according to claim 2 wherein the first circuit is mountedwithin a base unit.
 6. The system according to claim 5 wherein thedevice comprises a portable electric toothbrush.
 7. The system accordingto claim 5 wherein the device comprises a portable electric shaver. 8.The system according to claim 5 wherein the device comprises a portabletelephone.
 9. The system according to claim 5 wherein the devicecomprises a portable flashlight.
 10. The system according to claim 5wherein the device comprises a portable computer.
 11. The systemaccording to claim 5 wherein the device comprises a portable electricscrewdriver.
 12. A system for transferring electric power from a firstcircuit to a second circuit comprising;a. a first electric circuitcomprising a first coil coupled around a first magnetic core wherein thefirst coil is coupled to receive an AC power signal for forming avarying magnetic field, wherein the first magnetic core has a core gap,and the core gap having a first gap face and a second gap face whereinthe first gap face is substantially parallel to the second gap face andwherein the first gap face and the second gap face are spaced andaligned to minimize radiation of magnetic flux such that the firstmagnetic core forms a substantially closed magnetic path; and b. asecond electric circuit comprising a second coil coupled around a secondmagnetic core for receiving the varying magnetic field wherein thesecond magnetic core has a first end and a second end and wherein thesecond magnetic core is insertable into the core gap so that the firstend is aligned with the first gap face and the second end is alignedwith the second gap face to more fully close the magnetic path such thatthe radiation of the magnetic flux is minimized when the second magneticcore is aligned and also when separated from the first magnetic core.13. The system according to claim 12 wherein the second coil is coupledto rectifier for charging a battery coupled to the rectifier.
 14. Thecircuit according to claim 13 wherein the size of the core gap isminimized to reduce the radiation of magnetic flux and wherein thesecond magnetic core is sufficiently large to accommodate sufficientwindings of the second coil to charge the battery.
 15. The secondcircuit according to claim 14 wherein the core gap is substantially 1/2inch.